1. Field of the Invention
This invention relates to an instrument for measuring the frequency of a horizontal synchronizing signal particularly upon production of a horizontal synchronizing signal necessary for synchronous control of a multi-synchronism type display unit which receives a horizontal synchronizing signal and a vertical synchronizing signal from the outside to produce another horizontal synchronizing signal and another vertical synchronizing signal therefor.
2. Description of the Related Art
A horizontal synchronizing signal frequency measuring instrument is conventionally known and disclosed, for example, in Japanese Patent Laid-Open Application No. Heisei 4-97393. The horizontal synchronizing signal frequency measuring instrument disclosed in the document just mentioned is shown in FIG. 4.
Referring to FIG. 4, the horizontal synchronizing signal frequency measuring instrument shown includes a detection time window setting counter 12 which counts the number of pulses of a reference clock signal 11 produced by a reference clock generation circuit 10 to produce a detection window time signal 13, and a horizontal synchronizing signal counter 15 which counts the number of pulses of a horizontal synchronizing signal 14 inputted within the period of a detection window time of the detection window time signal 13. Thus, the horizontal synchronizing signal frequency measuring instrument calculates the horizontal synchronizing signal frequency from the detection window time and the counted pulse number of the horizontal synchronizing signal.
The conventional horizontal synchronizing signal frequency measuring instrument is disadvantageous in that, since each of the front edge and the rear edge of the detection window time includes a measurement error corresponding to one period of the horizontal synchronizing signal at the greatest, the frequency cannot be detected with a tolerance less than one period of the horizontal synchronizing signal, and consequently, a displacement is produced between the horizontal synchronizing signal received from the outside and the horizontal synchronizing signal produced in the inside of the instrument and this makes a displayed image blurred or dull.